This invention generally relates to multi-layer ceramic capacitors, and particularly relates to multi-layer ceramic capacitors having high reliability, high specific resistivities especially at high temperature and low dielectric loss at high frequencies, and including an inexpensive electrode therefore.
Recently, multi-layer ceramic capacitors are widely used to meet needs of small sized and large capacitance capacitors. Conventional electrolytic capacitors generally have large capacitances, but can not operate normally at a high frequencies. Therefore, in high frequency circuits, multi-layer ceramic capacitors are used at points where large capacitances are necessary. In general, multi-layer ceramic capacitors are produced by the step of co-firing internal electrodes and dielectric ceramics. Multi-layer ceramic capacitors of the high dielectric constant type conventionally include barium titanate (BaTiO.sub.3) based ceramics. Since the firing temperature of this material is high e.g., about 1300.degree. C., the internal electrodes of such capacitors should be made of materials such as platinum (Pt) and palladium (Pd), which are expensive.
Proposed multi-layer ceramic capacitors use BaTiO.sub.3 based materials which can be fired in atmosphere or ambient gas of low partial pressure oxygen and have internal electrodes made of a base metal such as nickel (Ni).
U.S. Pat. No. 3,920,781 discloses a method of manufacturing a multi-layer ceramic capacitor which uses acceptor dopant BaTiO.sub.3 based dielectric materials and which has an internal electrode made of a base metal such as nickel (Ni), cobalt (Co), or iron (Fe).
U.S. Pat. No. 4,115,493 discloses a multi-layer ceramic capacitor which uses dielectric materials made by doping or adding calcium (Ca) to a BaTiO.sub.3 based material. In this case, the ratio A/B is chosen to be 1.005-1.03 where the letter A denotes sum of the mole ratio of Ba and Ca while the letter B denotes the sum of the mole ratios of the other cations. In addition, this multi-layer ceramic capacitor has an internal electrode made of nickel.
In these multi-layer ceramic capacitors, since dielectric materials consists of BaTiO.sub.3 based material whose firing temperature is higher than the melting point of copper (Cu), a base metal such as nickel (Ni) is used as an internal electrode in place of Copper. A base metal such as nickel has a lower equilibrium oxygen partial pressure than that of copper which have the following disadvantages. When dielectric ceramics are fired together with internal electrodes made of a base metal such as nickel, the dielectric ceramics are exposed to the equilibrium oxygen partial pressure of the base metal at the firing temperature so that the dielectric ceramics tend to be reduced and to have low resistivities.
U.S. Pat. No. 4,752,858 discloses a multi-layer ceramic capacitor which uses dielectric materials of low temperature sintering ceramics including an oxide containing a component A and a component B, the component A being selected from a group I of lead, calcium, strontium, and barium, the component B being selected from a group II of magnesium, nickel, titanium, zinc, niobium, and tungsten, the component A including lead and at least one of the other substance in the group I, the component B including at least two of the substances in the group II, the ratio between values a and b being chosen so that a/b&gt;1.00, the internal electrode containing at least copper.
In this multi-layer ceramic capacitor, since the dielectric material consists of Pb based complex perovskite materials whose firing temperature is lower than 1080.degree. C., copper can be used as internal electrodes. In addition, since Pb based complex perovskite dielectric materials have higher dielectric constants and have lower dielectric losses than BaTiO.sub.3 based materials especially in high D.C. and A.C. bias voltage fields, multi-layer ceramic capacitors employing these dielectric materials have large capacities and low loss factors. But since the sintering bodies of these dielectric materials does not have sufficient density and have low specific resistivity at high temperature, the multi-layer ceramic capacitors do not have enough reliability in high temperature and in high humidity.